I think the idea is that any two people who are alive at the same time are of the same "generation."
Extreme example: Charles is born in 1914 and lives to be 100. Just before Charles dies in 2014, Joe is born.
Then Joe lives to be 100, dying in 2114.
Even though together their lives spanned 200 years (1914-2114), they were of the same "generation" because their lives overlapped.
If you are a JW, you are supposed to believe that this is what Jesus meant when he said, "This generation will by no means pass away..."
This is a far cry from the days when the WT said that people had to have been old enough to witness the events of 1914 with understanding in order to be a part of the 1914 generation.
Isn't "new light" grand?